The present invention refers to a method for the continuous casting of molten materials, and to the corresponding device for carrying out the said method, in particular usable in plants for the continuous casting of billets, blooms, slabs and the like, to improve the surface and internal quality of the cast product.
Continuous casting is a technique which is today extensively used in the production of metallic bodies having various shapes and sizes, such as blooms, slabs, billets, and the like. The aim is to achieve increasingly high speeds of vertical casting, which, however, leads to the accentuation of certain problems, such as the difficulty of obtaining a uniform distribution of the speeds throughout the cross section of the product being cast. In addition, the use of a discharging device immersed in the molten material does not facilitate the attainment of a uniform distribution of the said speeds on account of the turbulence caused in the metallic mass.
These elements may give rise to certain phenomena that damage the surface and internal quality of the finished products. Among other things, it is possible to cite as an example the lack of uniformity in the flow of liquid metal in the crystallizer, with consequent non-uniform solidification and possible tearing of the shell being formed. This danger is more accentuated, in particular, in the case of high rates of casting, at which the said shell is formed with a smaller thickness and/or a larger number of cracks is present on the surface or within the cast product, thus possibly causing disastrous leakage of liquid metal from the solidified shell of the ingot.
Another unfavourable phenomenon that may occur regards disturbances of the stability of the meniscus which may cause poor lateral lubrication and leads to the solidifying metal sticking to certain points of the crystallizer, so increasing the risk of tearing of the shell and the production of defects on its surface.
Another problem that may arise in this type of plant is linked to the longitudinal oscillation of the crystallizer, which is necessary to prevent the solidifying metal from sticking to the walls of the inner duct of the crystallizer and to facilitate flow of particular lubricants in the gap between the crystallizer and the solidifying shell. However, this oscillation may, in the presence of a disturbed meniscus, bring about deep and irregular markings on the surface of the shell of the cast product. The drawbacks listed above are of considerable importance both for the final quality of the cast product obtained and for the optimization of the production achievable from the casting system and for the cost of the subsequent transformation of the finished products.
In fact, to verify the defectiveness of the billets, blooms, slabs or similar cast pieces it is necessary to inspect them and possibly subject them to additional surface-conditioning treatments, this resulting in an increase in production costs and/or in a poorer quality of the end product.
Numerous solutions have been proposed in the past to overcome such problems. For example, particular covering powders are used for limiting oxidation of the molten bath and for lubricating the interface between the solidifying metal and the walls of the crystallizer in a more stable way, with consequent positive effects on the surface and internal quality of the cast product.
Another known solution consists of using particular discharging devices, set between the tundish and the crystallizer, whereby it is possible to control the flow of the liquid metal entering the crystallizer, so as to reduce the non-metallic inclusions in the cast product, at the same time favouring flotation of gases on the surface. In this way, there is a reduction in the disturbance of the meniscus, where the initial solidification occurs, and direct xe2x80x9chotxe2x80x9d flows of molten metal are avoided, which could lead to the partial re-melting of some areas of the shell that is forming. Attempts to improve the fluid-dynamic conditions in the crystallizer regard the use of particular xe2x80x9cducts or tanksxe2x80x9d made of refractory material and set immediately upstream of the crystallizer with the purpose of removing the meniscus of the liquid metal from the area of start of solidification, thus limiting the possibility of drawing particles of refractory material or dross into the solidifying metal, and favouring uniformity of the rate of flow of the metal and of heat exchange between the cast product and the crystallizer, especially in the area of initial solidification, which normally takes place in the area of joining between the refractory material of the walls of the xe2x80x9ctankxe2x80x9d and the contiguous edge of the cooled metal crystallizer, referred to as xe2x80x9ctriple pointxe2x80x9d.
The situation in this area proves very delicate because the molten metal tends to adhere to the refractory material, which is colder on account of the vicinity of the copper cooled by forced circulation. Consequently, in this area surface defects or failures arise in the bodies produced.
To overcome such a problem, from the U.S. Pat. Nos. 5,027,887, 5,045,276, and 4,130,423 it is known that gases may be used, such as nitrogen or argon, or solid lubricants, which are injected at the said joint to form a protective layer. However, in this type of solution, the liquid metal, which is considerably heavier than the lubricant and the gas, frequently manages to tear the protective layer and to come into contact even so with the walls of the xe2x80x9ctankxe2x80x9d, which are made of refractory material.
The U.S. Pat. Nos. 5,494,095 and 5,379,828 propose the solution of setting, between the xe2x80x9ctankxe2x80x9d made of refractory material and the crystallizer, an insert consisting of a material having a thermal and electrical conductivity lower than that of the material of which the crystallizer is built, so that the molten metal will start to solidify at the insert itself. The joint between the insert and the refractory material of the tank is heated by means of an alternating electromagnetic field.
A similar solution, but without heating of the joint between the refractory material and the intermediate insert is proposed by the U.S. Pat. No. 4,773,469.
In continuous casting, in particular of in the casting of thin slabs (i.e., just a few centimeters thick) or of strip Oust a few millimeters thick), it has been proposed to use electromagnetic fields in order to obtain confinement of the molten metal (see, for example, the U.S. Pat. Nos. 4,353,408 and 5,513,692).
Up to now, the systems referred to above have not yielded satisfactory results or have proven too costly to implement. For this reason, the present invention proposes to overcome the drawbacks discussed above presented by the known systems of the state of the art.
A primary purpose of the present invention is to overcome the problems referred to above by providing a method of continuous casting presenting high efficiency, productivity and reliability.
One aim of the present invention is to improve the surface quality of continuously cast products, combining this improvement with an increase in the casting speed to obtain a consequent increase in productivity.
A further purpose of the present invention is to protect the triple point from direct contact with the liquid metal, preventing cooling of the metal in that area.
A further purpose of the present invention is to reduce the surface wear of the refractory material with which the tank or duct upstream of the crystallizer is lined. These purposes are achieved by a device for the continuous casting of molten materials which comprises a first duct, designed to receive molten material, set in a substantially vertical position, a second duct designed to cool the molten material, set in a position lower than that of the said first duct, the said first and second ducts being axially aligned and connected operatively to define a channel designed to enable passage of said molten material, means of injection of the molten material into said channel, and electromagnetic means arranged around at least one stretch of said channel and coaxially thereto and designed to generate magnetic forces operating on said molten material, the said device being characterized in that the said electromagnetic means are made up of a plurality of coils of electrically conductive material and a ferromagnetic core, which can be electrically supplied and are designed to produce a magnetic flux in a direction longitudinal to the channel itself, producing a set of forces acting on said molten material directed orthogonally to the direction of said magnetic flux to keep the outer surface of said molten material detached from the walls of said channel for a substantial stretch of its length.
Thanks to this arrangement, the productivity of the machine increases considerably, producing a more homogeneous cast product having high surface finish, with a consequent reduction in production costs as compared to known continuous-casting devices of the past.
According to a further aspect of the invention, a method is envisaged for continuous casting of molten materials, in particular of metallic bodies such as blooms, slabs, billets, and the like, which, comprises the following steps:
a) continuous pouring of the molten material into a funnel by means of a discharging device until a level corresponding to the covering of the said discharging device is reached;
b) formation of a protective layer of dross on the top surface of the molten material;
c) excitation, with alternating current, of a plurality of electromagnetic coils, generating a longitudinal magnetic flux inside the duct of the funnel made of refractory material containing the molten material;
d) detachment of the external surface of the molten material from the inner wall of said duct so as to form a free space on the entire perimeter of the molten material;
e) advance of the molten material along the duct and in the direction of a second duct of the crystallizer, keeping the thickness of said free space constant along the entire perimeter of the molten material;
f) start of the process of solidification of the molten material just below the area of joining between the funnel and crystallizer equipped with a forced cooling system, with start of formation of a solid superficial layer of the cast material;
g) continuation of solidification of the material during advance of the piece inside the duct of said crystallizer; and
h) extraction of the solidified material from the casting device by appropriate means of extraction.
Thanks to this method, any phenomena of sticking of the cast material to the walls of the duct or crystallizer are prevented, and protection of the surface of the refractory material lining the inside of duct or tank is guaranteed.